Electrostatographic printing machines typically use electrical contacts and devices. In electrostatographic printing devices commonly used today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image contained within the original document. Alternatively, a light beam may be modulated and used to selectively discharge portions of the charged photoconductive surface to record the desired information thereon. Typically, such a system employs a laser beam. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developer powder referred to in the art as toner. Most development systems employ a developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the latent image areas of the photoconductive insulating area to form a powder image on the photoconductive area. This toner image may be subsequently transferred to a support surface, such as copy paper, to which it may be permanently affixed by heating or by the application of pressure, to form the desired copy.
In commercial applications of such printing machines, it is necessary to distribute power and/or logic signals to various sites within the machines. Traditionally, this has required conventional wires and wiring harnesses in each machine to distribute power and logic signals to the various functional elements in an automated machine. In such distribution systems, it is necessary to provide electrical connectors between the wires and components. In addition, it is necessary to provide sensors and switches, for example, to sense the location of copy sheets, documents, etc. Similarly, other electrical devices such as interlocks, and the like are provided to enable or disable a function. These electrical devices usually operate at low power, typically electronic signal potentials of up to 5 volts and at currents in the milliamp regime. Further, many commercial applications employ electrical contact components and related devices that require currents in the regime of 1-100 amps and voltages greater than 5 volts.
Further, conventional electrical devices employ mating pairs of electrical contacts which are made from metal, or, base metal overplated with additional metals or metal alloys. High contact loads, for example 100 grams to 500 grams, are typically required with these metal contact systems which contribute to long term wear out of mechanical springs, etc., and to the mechanical and tribological deterioration of the contact surfaces by abrasion, wear, crushing, deformation, and the like.
Conventional electrical components are disclosed in U.S. Pat. No. 5,599,615 to Swift et al., U.S. Pat. No. 5,270,106 to Orlowski et al., U.S. Pat. Nos. 5,139,862, 5,250,756 to Swift et al., and Swift et al., “Static Eliminator Brush Structure,” XEROX DISCLOSURE JOURNAL, Vol. 10, No. 2, page 109-110 (March/April 1985).
U.S. Pat. No. 5,744,090 to Jones et al., discloses a process for the manufacture of conductive fibers usable in electrostatic cleaning devices, where magnetic, electrically conductive material is used.
In electophotography, there is a common need for inexpensive, easily fabricated, resistive polymeric matrix compositions, such as films or resins, etc., such as for use in electrical contacts and devices, which vary over a substantial resistance range. The resistance of the films is changed by varying the quantity of conductive material dispersed in an insulating binder. A greater resistance is achieved by lower loadings of the conductive material, where small decreases in loading of conductive materials at the percolation threshold cause dramatic increases in resistance. Typically, such materials have a surface resistivity in the range of about 102 ohms/square to 108 ohms/square and a thickness in the range of about 1.0 micron to 500 microns. For example, thin films having resistivities controlled to fall within such ranges, are used to overcoat other materials to comprise a multiple-layer component. As a result, the surface layer of such a coated component exhibits static discharge, electrostatic bleed-off behaviors, and other similar characteristics. However, it is difficult to control and maintain films or resin based composites associated with known resistivity values or resistivity ranges precisely due to sudden resistance changes that are caused by improper selection of material compositions used to make the subject films or resin composites and which occur at specific percolation thresholds. Dramatic increases in resistance are observed when conductive particles are incorporated into such composite materials, which render material composites conductive.
Conductive particles have been loaded in composites in varying quantities to control resistance levels. Light loadings of conductive particles to insulating host polymers have been attempted to eliminate dramatic increases in resistance at specific percolation thresholds. However, the ability to precisely control material properties of such a composite is hampered by inhomogeneities that result from poor quality dispersion of small filler material amounts to a host matrix polymer. To reduce this effect, less conductive filler materials have been used at relatively high loadings. For example, different metal, metal oxide containing particles and carbon black particles have been used in attempts at achieving tightly controlled electrical resistivities. However, high loadings of particles in a film are known to make the film hard or brittle.
An example of the need for resistive compositions with controlled electrical properties can be found in corona charging devices, such as scorotrons. The flat scorotron is a current charging device based on a concept in European Patent Publication No. 0-274-895 to Gundlach et al. The device comprises a set of thin conductive lines deposited on a substrate and is used to replace the free-standing corona wire in a typical electrophotographic device. A flat scorotron has a number of advantages over other corona charging devices, such as being easier to clean, less likely to break because of paper misfeed or cleaning, and inexpensive to produce. However, the device suffers from a number of problems. Any differences in the microstructure of the pins causes each pin to form a corona at a slightly different voltage. Once a corona forms at the end of a pin, the voltage on the array of pins drops, because the corona sustaining voltage is less than the corona onset voltage. The drop in voltage prevents other pins from, forming a corona. This self-limiting behavior can be overcome by including current limiting resistances between each pin and the bus bar, which supplies the high voltage to all of the pins in the array. However, it is difficult to control the individual distributed resistances between the pins and bus, because the required resistivity for such devices is at the edge of the percolation threshold for most materials. Any small, local changes in composition result in large changes in resistivities making it difficult to obtain a precisely controlled and uniform resistivity.
A general example of the need for resistive matrix compositions can be found in simple voltage sensors for electrostatically charged surfaces. A high voltage sensor fabricated with a resistive film bleeds only a small quantity of charge from a surface leaving the charge density nearly unchanged. The need for resistive compositions also can be found in document sensing devices in xerographic copying machines. As a document or paper passes between an electrical contacting brush and a resistive film, the resistance of the circuit is changed. A sensing circuit will produce a signal indicative of the presence and position of the paper and the document path may be corrected. See H. Rommelmann et al, Xerox Disclosure Journal 12(2) 81-2 (1987).
Fibers having electrically conductive properties have been used to achieve conductive compositions. For example, U.S. Pat. No. 4,491,536 to Tomoda et al. discloses a composition comprising a fluoroelastomer and carbon fibers having a length of 0.1 millimeters to 5 millimeters. A volume “resistivity” of 10−1 ohm-cm to 1013 ohm-cm can be achieved with that composition. A slight increase in the loading of carbon fiber may produce a dramatic change in volume resistivity of as much as 12 orders of magnitude difference. Thus, slight inconsistencies in the composition may lead to large changes in resistivity, especially in compositions having about 15% to 25% by weight fibers.
U.S. Pat. No. 4,569,786 to Deguchi discloses an electrically conductive composition comprising metallic and carbon fibers dispersed in a thermoplastic resin. The metallic and carbon fibers have a length of from 0.5 mm to 10 mm and are provided to impart a high degree of conductivity to the composition.
U.S. Pat. No. 3,406,126 to Litant discloses a conductive synthetic resin composition containing carbon filaments having a length of ¼ inch to ¾ inch in length.
U.S. Pat. No. 4,810,419 to Kunimoto et al. discloses a shaped electroconductive aromatic imide polymer article comprising an aromatic imide polymer matrix and 10% to 40% by weight of 0.05 mm to 3.0 mm long carbon fibers.
In general, desired resistivity of a conductive composition may be achieved by controlling the loading of the conductive particles and/or other filler materials. Very small changes in loading of conductive filler materials near a threshold value at which conduction occurs, i.e., percolation threshold, can cause dramatic changes in a composition's conductivity. Furthermore, differences in any of the fibers, filler materials, etc., such as particle form, size and shape can cause wide variations in conductivity at even a constant weight loading. Moreover, the percolation threshold approach requires sufficiently high concentrations of conductive particles, wherein such concentration levels allow for conductive particle-to-particle contacts, which span the thickness of the composite, resulting in a burst of conductivity at the point where a first continuous particle chain is formed. As conventionally known in the art, conductive filler materials generally have D.C. volume resistivity values of less than about 10−4 to 10−6 ohm-cm, while insulating materials, on the other hand, generally have resistivity values of greater than 1013 ohm-cm to 1014 ohm-cm. “Controlled conductivity” materials, of intermediate resistivities, may have resistivity values ranging from about 10−6 ohm-cm to about 1013 ohm-cm.
In these and other references, the emphasis has been primarily on achieving highly conductive compositions, where a resulting resistivity is typical of that at, or slightly above, a percolation threshold values for specific material compositions, which are achieved by the use of highly conductive fiber or fillers of the prior art. However, maintaining a resistivity value slightly beyond such a percolation limit is difficult to control accurately and precisely, based upon variations in compositions used and inconsistencies of the materials contained therein, such as even small differences in final loading of fibers, fillers etc. In addition, small variations of fiber loadings within a component may lead to significant performance variations within that component leading to localized problems such as degraded performance in random, small regions of the component. Moreover, depending on the materials used in a composition, materials may become brittle due to slight variations resulting in changes in the mechanical properties associated with such compositions. It is further known that low conductivity values are required, and often necessary, for electrophotographic image development systems, wherein the intrinsic electrical conductivity of materials used in a composition make it extremely difficult to achieve predictable and reproducible conductivity values.
There continues to be a need for materials having intermediate and stable resistivity suitable for use in the present invention that can be precisely controlled to avoid inhomogeneities in resistivity within such polymeric-based compositions, such as for resins, films, etc. which can be formed from low cost and commercially available materials.